I was referring to the vehicle side of the harness, not the head unit side. Considering he's installed an aftermarket unit there should be one in there already unless he chopped the plug off and spliced in directly.
This is a very bad idea. At the very least use some insulated crimp connectors. These heat shrink/solder connectors are great and you can melt them in place with a heat gun or even just a lighter.
https://www.amazon.com/gp/product/B07HCNTZ2Z
If you have no plans to add any external amps or subs one of these would probably be a great option.